1. Field of the Invention
The present invention relates to testing of information communication systems. More particularly, the present invention relates to a system and method for locating and determining discontinuities and estimating loop loss in a communications medium using frequency domain correlation.
2. Background Information
A time domain reflectometer (TDR) operates by abruptly stimulating an object under test, and subsequently, over time, recording the elicited responses. Typically, the subject object is a medium designed for the propagation of energy from point to point in some form, such as, for example, sound, light or electricity. One type of TDR can analyze the electrical propagation characteristics of extended lengths of twisted pairs of wires, as are commonly used to transport telecommunications signals. Such a TDR is designed to interface with twisted pair wire lines that are deployed as a subscriber loop plant (the pair of leads over which each subscriber has traditionally been provided fixed location telephone service) from the service provider's end.
In operation, such a TDR emits a probe signal of an appropriate kind for the subject telecommunications medium. In other words, in the TDR application, the transmitter injects a stimulus into the transmission medium, such as a transmission line. Non-uniformities in the telecommunications medium exist, and probe signal energy is lost progressively as a function of distance traveled in the medium, due to the unavoidable dissipative characteristics of the medium material. Any non-uniformity encountered as the probe signal stimulus propagates along the medium results in at least a partial reversal of probe signal energy flow, thus producing a return or echo signal that propagates back toward the stimulus source. These echoes, or reflections, are essentially returning signatures of aberrations in an otherwise continuous and uniform telecommunications transmission medium. In other words, a reflected and a forward signal are generated in response to a change of the characteristic impedance—a discontinuity—along the transmission line. Additionally, part of the generated forward signal will be reflected back if more discontinuities exist. The TDR records echoes or reflections, if any, from the moment of probe signal emission, as a function of time as the echoes arrive back at the source. Recording these echoes with respect to time provides a signature of the medium that reveals non-uniformities created either intentionally or by accident, along the propagation path. In other words, the reflected signals are received by a receiver and analyzed to impute the transmission line topology. In addition, the distance from the observation point to any particular aberration can be imputed from the corresponding return delay time relative to the probe signal stimulus, if the propagation speed for the medium is known. Thus, a TDR can provide a convenient means of identifying the type of, and distance to, a particular discontinuity or defect.
The probe signal, or stimulus, used for TDR is generally a uni-polar pulse. An example of such a uni-polar stimulus signal can be pt(t)=0.5*(1−cos(2πfct)), 0≦t≦2 μsec, where fc=500 kHz, although other uni-polar pulses can be used, as well as any center frequency. Using such a stimulus signal, large amounts of DC and low frequency components exist. This can pose a problem for the TDR implementation for twisted-pair and similar dispersive channels. In twisted-pair channels, the higher frequency components will be attenuated more than the lower frequency components. This is referred to as attenuation distortion. Additionally, the higher frequency components will travel faster than the lower frequency components. This is referred to as dispersion. Since a large number of frequency components exist, the overall effect is that the reflected signal becomes smeared out, making it difficult to determine the starting time of the reflection and, hence, the location of the discontinuity. Similarly, since the attenuation of the received signal is comprised of compound effects from components from a large frequency range, especially the low frequency range, it becomes difficult to determine the round-trip loss of the communication channel.
Thus, a uni-polar stimulus can be comprised of a very broad set of frequency components. It is well known that the phase constant is a function of frequency and that different frequency components propagate at different phase velocities in a dispersive transmission line, such as practical twisted-pair subscriber line. The different phase velocities at different frequencies lead to dispersion of signal energy and to inter-symbol interference. In the conventional TDR application, the locations of the discontinuities are obtained by multiplying the one-half of round-trip time to the considered discontinuity with a phase velocity. Since the stimulus is a broadband signal, it is extremely difficult to find a phase velocity that is suitable for all different lengths and structures of transmission lines. For example, a velocity factor of 0.67 of the light speed may be suitable for a short line, while 0.58 or lower may be more appropriate for a long line. The reason for the difficulty is that the extremely high attenuation of the high frequency components for the long line leads to components at lower frequency band dominating the reflected signal.
In conventional TDR implementations, some efforts have been made to address the issues with dispersion. For example, a high-pass filter can be used in the receiver. Use of the high-pass filter can result in the attenuation of frequency components up to a certain frequency (depending on the high-pass filter used). However, generally large amounts of low-pass frequency components will still exist. The dispersion in the twisted-pair channel can then still make localization of a discontinuity difficult. Thus, the attenuation distortion and dispersion can adversely affect the accuracy of traditional TDR measurements.